Grease composition and rolling device

ABSTRACT

As a grease composition maintaining a low torque over a wide temperature range and further hardly inducing white texture flaking, the present invention provides a grease composition comprising a base oil having a dynamic viscosity at 40° C. of 20 to 50 mm 2 /s, a urea compound as a thickening agent in an amount of 8 to 30% by weight with respect to total weight of the grease, at least one antirust additive selected from carboxylic acids, carboxylate salts, and ester-based antirust additives in an amount of 0.1 to 10% by weight per single additive with respect to total weight of the grease and in an amount of 0.1 to 15% by weight in total of the additive with respect to total weight of the grease, and a rolling device comprising the grease composition which works with a low torque, hardly induces white texture flaking, and is excellent in durability.

TECHNICAL FIELD

The present invention relates to a grease composition particularlyexcellent in anti-flaking performance and anti-seizure performance in awide temperature range of from a low temperature to a high temperatureand having a low torque performance. Moreover, the invention relates toa rolling device such as a rolling bearing, a ball screw device, or alinear guide device into which such a grease composition is charged.

BACKGROUND ART

In rolling bearings for use in motors and the like and ball screwdevices and linear guide devices for use in machine tools and the like,a small torque, a maintenance-free property (durability for a longperiod of time), and the like characteristics are required for enhancingworking efficiency. In these rolling devices, grease compositions havebeen generally used for lubrication. In order to satisfy suchrequirements, a lithium soap grease (e.g., “Multemp SRL” manufactured byKyodo Yushi Co., Ltd.) containing a low viscosity ester oil-basedsynthetic oil has been used. However, such a grease composition achievesa low torque but ester-based synthetic oil are generally insufficient inheat resistance and are problematic in seizure life.

Moreover, rolling bearings have widely been used in engine auxiliariesof automobiles, such as an alternator, but they have many chances tocome into contact with water. Among the machine tools, some of them comeinto contact with water. Therefore, these rolling devices should beexcellent in antirust properties. Thus, a sulfonate salt excellent inantirust properties is frequently added to a grease composition (e.g.,see Japanese Patent Unexamined Publication JP-A-7-179879). However, thesulfonate salt has a problem that it promotes generation of hydrogenowing to degradation of a grease composition and is apt to induceflaking involving white texture change caused by the hydrogen(hereinafter referred to as “white texture flaking”).

The invention is conducted in consideration of the above circumstancesand an object of the invention is to provide a grease compositionparticularly maintaining a low torque over a wide temperature range offrom a low temperature to a high temperature and further suppressinggeneration of white texture flaking as well as a rolling device intowhich the above grease composition is charged and which works with a lowtorque, hardly induces white texture flaking, and is excellent indurability.

DISCLOSURE OF THE INVENTION

In order to achieve the above object, the present invention provides agrease composition comprising a base oil having a dynamic viscosity at40° C. of 20 to 50 mm²/s; a urea compound as a thickening agent in anamount of 8 to 30% by weight with respect to total weight of the grease;at least one antirust additive selected from carboxylic acids,carboxylate salts, and ester-based antirust additives in an amount of0.1 to 10% by weight per single additive with respect to total weight ofthe grease and in an amount of 0.1 to 15% by weight in total of theadditive with respect to total weight of the grease, and a rollingdevice into which the above grease composition is charged.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating one embodiment of a rollingbearing which is one of the rolling devices of the invention,

FIG. 2 is a graph illustrating the relationship between dynamicviscosity of a base oil and an flaking life ratio obtained in Examples,

FIG. 3 is a graph illustrating the relationship between dynamicviscosity of a base oil and dynamic torque obtained in Examples,

FIG. 4 is a graph illustrating the relationship between dynamicviscosity of a base oil and a seizure life ratio obtained in Examples,

FIG. 5 is a schematic block diagram illustrating a test apparatus foruse in measurement of flaking generation probability,

FIG. 6 is a graph illustrating the relationship between an amount ofzinc naphthenate added and flaking generation probability or a rustevaluation score obtained in Examples, and

FIG. 7 is a graph illustrating the relationship between an amount of anorganometallic salt (ZnDTC) and flaking generation probability orseizure life time obtained in Examples.

Numeral 1 represents an inner ring, 2 represents an outer ring, 3represents a ball, 4 represents a holder, and 5 represents a sealingmember in the figures.

BEST MODE FOR CARRYING OUT THE INVENTION

The following will explain the present invention in detail.

[Grease Composition]

(Base Oil)

In the invention, the base oil for use in the grease composition is notparticularly limited except that the dynamic viscosity at 40° C. is from20 to 50 mm²/s. In order to secure torque performance at a lowtemperature and more surely avoid seizure which occurs since an oil filmis hardly formed at a high temperature, the dynamic viscosity of thebase oil at 40° C. is preferably from 25 to 50 mm²/s.

As specific examples usable as the base oil, there may be mentionedmineral oil-, synthetic oil-, or natural oil-based lubricating oil andthe like. As the mineral oil-based lubricating oil, use can be made ofthose obtained by purifying mineral oil by suitably combiningdistillation under reduced pressure, oil-deasphalting, solventextraction, hydrogenolysis, solvent dewaxing, sulfuric acid washing,clay purification, hydrogenative purification, and the like. As thesynthetic oil-based lubricating oil, there may be mentioned hydrocarbonoil, aromatic oil, ester-based oil, ether-based oil, and the like. Asthe hydrocarbon oil, there may be mentioned normal paraffins,isoparaffins, poly-α-olefins such as polybutene, polyisobutyrene,1-decene oligomers, and cooligomers of 1-decene with ethylene,hydrogenated products thereof, and the like. As the aromatic oil, theremay be mentioned alkylbenzenes such as monoalkylbenzenes anddialkylbenzenes, alkylnaphthalenes such as monoalkylnaphthalenes,dialkylnaphthalenes, and polyalkylnaphthalene, and the like. As theester-based oil, there may be mentioned diester oil such as dibutylsebacate, di-2-ethylhexyl sebacate, dioctyl adipate, diisodecyl adipate,ditridecyl adipate, ditridecyl glutamate, and methyl acetylcinolate,aromatic ester oil such as trioctyl trimellitate, tridecyl trimellitate,and tetraoctyl pyromellitate, further, polyol ester oil such astrimethylolpropane caprilate, trimethylolpropane pelargonate,pentaerythritol-2-ethylhexanoate, pentaerythritol pelargonate,furthermore, complex ester oil which are oligoesters of polyhydricalcohols with mixed fatty acids of dibasic acids and monobasic acids,and the like. As the ether-based oil, there may be mentioned polyglycolssuch as polyethylene glycol, polypropylene glycol, polyethylene glycolmonoethers, and polypropylene glycol monoethers, phenyl ether oil suchas monoalkyl triphenyl ether, alkyl diphenyl ether, dialkyl diphenylether, pentaphenyl ether, tetraphenyl ether, monoalkyltetraphenyl ether,and dialkyltetraphenyl ether, and the like. As the other syntheticlubricating base oil, there may be mentioned tricresyl phosphate,silicone oil, perfluoroalkyl ethers, and the like. As the naturaloil-based lubricating base oil, there may be mentioned oil-and-fat-basedoil such as beef tallow, lard, soybean oil, rapeseed oil, rice bran oil,coconut oil, palm oil, palm kernel oil and hydrogenation productsthereof. Among these lubricating oil, in view of the use in a widetemperature range of from a low temperature to a high temperature,ester-based synthetic oil, synthetic hydrocarbon oil, ether-basedsynthetic oil, and the like are preferred.

The lubricating oil mentioned above can be used singly or as a mixtureobtained by suitably combining them and they are adjusted to a preferreddynamic viscosity as mentioned above.

(Thickening Agent)

Any compound can be used without particular limitation as far as it is aurea compound However, in view of acoustic performance, long-termstability, and the like, a mixture of diurea compounds represented bythe following general formulae (1) to (3) is preferred. In thisconnection, the blending amount of the thickening agent is from 8 to 30%by weight with respect to total weight of the grease. When the amount isless than 8% by weight, the form as grease cannot be formed ormaintained and when it exceeds 30% by weight, a low torque performancecannot be realized.

Wherein R₁ is a cyclohexyl group or an alkylcyclohexyl group having 7 to12 carbon atoms, R₂ is a divalent aromatic ring-containing hydrocarbongroup having 6 to 15 carbon atoms, and R₃ is an alkyl group having 8 to20 carbon atoms.

Moreover, at the mixing, the above diurea compounds are mixed so that aratio of [number of moles of R₁/(number of moles of R₁+number of molesof R₃)] becomes from 0.1 to 1.0. When the value is less than 0.1, greaseleakage increases and there is a possibility that the seizure life maybe shortened. The ratio is preferably from 0.2 to 0.9.

(Antirust Additive)

In order to impart antirust performance without generating white textureflaking, at least one antirust additive selected from carboxylic acids,carboxylate salts, and ester-based antirust additives is blended intothe grease composition. These antirust additives do not promotegeneration of hydrogen involved in grease degenerative decompositionunlike sulfonate salts and can suppress the generation of white textureflaking. The amount of the antirust additive to be added is from 0.1 to10% by weight per single additive with respect to total weight of thegrease. When the amount is less than 0.1% by weight, impartment of theantirust performance is insufficient and when the amount exceeds 10% byweight, grease is softened and grease leakage tends to occur. In view ofsufficient impartment of the antirust performance and grease leakage,the amount to be added is preferably from 0.25 to 5% by weight.Furthermore, total amount of the antirust additives is from 0.1 to 15%by weight.

Among the carboxylate salts, naphthenate salts are preferred. Thenaphthenate salts are not particularly limited as far as they aresaturated carboxylate salts having a naphthene base. For example, theremay be mentioned saturated monocyclic carboxylate salts(C_(n)H_(2n−1)COOM), saturated polycyclic carboxylate salts(C_(n)H_(2n−3)COOM), aliphatic carboxylate salts (C_(n)H_(2n+1)COOM),and derivatives thereof. Moreover, as the monocyclic carboxylate salts,compounds represented by the following general formulae (4) and (5) canbe exemplified:

Wherein R₄ represents a hydrocarbon group and specifically, an alkylgroup, an alkenyl group, an aryl group, an alkaryl group, or an aralkylgroup, or the like may be mentioned; and M represents a metal elementand specifically Co, Mn, Zn, Al, Ca, Ba, Li, Mg, Cu, or the like. Thesenaphthenate salts may be used singly or as a suitable combinationthereof.

Further, as a carboxylic acid salt, succinic acid derivate is alsopreferable. As this succinic acid derivate, succinic acid, alkylsuccinicacid, alkylsuccinic half ester, alkenylsuccinic acid, alkenylsuccinichalf ester, succinic acid imido can be raised. These succinic acidderivates can be used singly or as a mixture obtained by suitablycombining them.

(Other Additives)

To the grease composition, if necessary, various additives hithertoknown can be added. Among them, organometallic salts are additiveseffective for improving flaking performance. Among the organometallicsalts, dialkyl dithiocarbamic acid(DTC)-based compounds shown in thefollowing general formula (6) and dialkyl dithiophosphoricacid(DTP)-based compounds shown in the following general formula (7) canbe suitably used:

Wherein M represents a metal species and specifically Sb, Bi, Sn, Ni,Te, Se, Fe, Cu, Mo, or Zn is used; R₅ and R₆ may be the same ordifferent from each other and each represents an alkyl group, acycloalkyl group, an alkenyl group, an aryl group, an alkylaryl group,or an arylalkyl group. Particularly preferred groups include a1,1,3,3-tetramethylbutyl group, a 1,1,3,3-tetramethylhexyl group, a1,1,3-trimethylhexyl group, a 1,3-dimethylbutyl group, 1-methylundecanegroup, a 1-methylhexyl group, a 1-methylpentyl group, a 2-ethylbutylgroup, a 2-ethylhexyl group, a 2-methylcyclohexyl group, a 3-heptylgroup, a 4-methylcyclohexyl group, an n-butyl group, an isobutyl group,an isopropyl group, an isoheptyl group, an isopentyl group, an undecylgroup, an eicosyl group, an ethyl group, an octadecyl group, an octylgroup, a cyclooctyl group, a cyclododecyl group, a cyclopentyl group, adimethylcyclohexyl group, a decyl group, a tetradecyl group, a docosylgroup, a dodecyl group, a tridecyl group, a trimethylcyclohexyl group, anonyl group, a propyl group, a hexadecyl group, a hexyl group, aheneicosyl group, a heptadecyl group, a heptyl group, a pentadecylgroup, a pentyl group, a methyl group, a tert-butylcyclohexyl group, atert-butyl group, a 2-hexenyl group, a 2-methallyl group, an allyl group(changed from “an aryl group” since it is overlapped with the above arylgroup), an undecenyl group, an oleyl group, a decenyl group, a vinylgroup, a butenyl group, a hexenyl group, a heptadecenyl group, a tolylgroup, an ethylphenyl group, an isopropylphenyl group, atert-butylphenyl group, a sec-pentylphenyl group, an n-hexylphenylgroup, a tert-octylphenyl group, an isononylphenyl group, ann-dodecylphenyl group, a phenyl group, a benzyl group, a 1-phenylmethylgroup, a 2-phenylethyl group, a 3-phenylpropyl group, a1,1-dimethylbenzyl group, a 2-phenylisopropyl group, a 3-phenylhexylgroup, a benzhydryl group, a biphenyl group, and the like and also thesegroups may have an ether bond.

Moreover, as the other organometallic salts, use can be made of ashlessdithiocarbamates such as methylene-bis-alkyl dithiocarbamate representedby the following general formula (8):

Wherein R₇ and R₈ each represents a hydrocarbon group having 1 to 18carbon atoms and R₇ and R₈ may be the same or different from each other.

The above organometallic salts may be used singly or as a combination oftwo or more thereof. In this connection, no limitation exists for thecombination. Moreover, the organometallic salt is added in an amount of0.1 to 10% by weight with respect to total weight of the grease in thecase that it is used singly. The organometallic salt has an action offorming a reaction film in minute spaces to suppress the white textureflaking but this action is not sufficiently exhibited when the amountadded is less than 0.1% by weight. On the other hand, when the amountadded exceeds 10% by weight, there is a possibility of deterioratingseizure performance through induction of excessive reaction between theorganometallic salts. Furthermore, the organometallic salt is expensiveand hence the case is not preferred also economically. Moreover, in thecase that the organometallic salts are used in combination, the amountof each organometallic salt is from 0.1 to 10% by weight as in the caseof single use but the total amount is preferably 15% by weight or less.When the amount added is more than 15% by weight, blending ratios of thebase oil, the thickening agent, and the antirust additive relativelydecrease and thus respective effects become insufficient.

(Production Process)

The process for producing the grease composition is not particularlylimited. An antirust additive and furthermore, if necessary, variousadditives such as organometallic salts and ashless dithiocarbamate maybe added to a grease composition obtained by reacting a thickening agentin the base oil and the whole may be thoroughly mixed and homogeneouslydispersed. At the time when the treatment is conducted, heating is alsoeffective.

Moreover, consistency of the above grease composition is preferably NLGI(National Lubricating Grease Institute) No. 1 to 3.

(Rolling Device)

The invention relates to a rolling device into which the above greasecomposition is charged. The rolling device is not limited and a rollingbearing, a ball screw device, a linear guide device, and the like can beexemplified. With regard to any rolling devices, the constitution is notlimited and it may be known one. For example, as a rolling bearing, aball bearing shown in FIG. 1 can be exemplified. The rolling bearingshown in the figure has a constitution wherein there are provided anouter ring 2 having an outer ring raceway 2 a on its inside surface, aninner ring 1 having an inner ring raceway 1 a on its outside surface, aplurality of rolling elements, balls 3 provided between the outer ringraceway 2 a and the inner ring raceway 1 a in a freely rollable manner,and a holder 4 which holds a plurality of the balls 3 in a freelyrollable manner, the above grease composition G is charged into a space6 between the outer ring raceway 2 a and the inner ring raceway 1 a, anda space between the inner ring 1 and the outer ring 2 is sealed by asealing member 5 fixed to a seal groove 2 b of the outer ring 2.

In this connection, the amount of the grease composition to be chargedis suitably selected according to the kind of rolling devices.

EXAMPLES

The following will specifically describe the invention with reference toExamples and Comparative Examples but the invention is not limitedthereto. Examples 1 and 2, Comparative Examples 1 to 6

Test greases were prepared with formulations shown in Table 1. Withregard to Examples 1 and 2 and Comparative Examples 2 to 6, a base oilmixed with a diisocyanate was reacted with the same base oil mixed withan amine and the whole was stirred under heating to obtain a semi-solidmaterial, an antirust additive (zinc naphthenate, a succinate ester,barium sulfonate) and/or an organometallic salt (zincdialkyldithiocarbamate: ZnDTC, zinc dialkyldithiophosphate: ZnDTP),which were already solved in the base oil, were added to the semi-solidmaterial, and the whole was thoroughly stirred and, after gradualcooling, was passed through a roll mill to obtain a grease. With regardto Comparative Example 1, stearic acid and lithium hydroxide werereacted in a base oil to form a lithium soap, barium sulfonate was addedafter cooling to room temperature, and the whole was thoroughly stirredand then was passed through a roll mill to obtain a grease. Thereafter,test greases were subjected to (1) white flaking life test, (2) dynamictorque test, and (3) seizure life test. TABLE 1 Comparative ComparativeComparative Comparative Comparative Comparative Example 1 Example 2Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Thickeningdiurea compound Li soap diurea compound agent Amount of 13% by 20% by11% by 13% by weight 20% by weight thickening weight weight weight agentR₁/(R₁ + R₃) 0.3 1.0 — 0.3 1.0 Base oil PAO PAO + ether ester oil PAOPAO + ether oil oil Dynamic 20 to 50 20 to 50  26  30  15  60  15  60viscosity of base oil Zinc 1% by 2% by — — 1% by 1% by 2% by 2% bynaphthenate weight weight weight weight weight weight Succinate 1% by 2%by — — 1% by 1% by 2% by 2% by ester weight weight weight weight weightweight ZnDTC 1% by — — — 1% by 1% by — — weight weight weight ZnDTP — 1%by — — — — 1% by 1% by weight weight weight Barium — — 1% by 2% by — — —— sulfonate weight weight Mixed 240 to 250 260 to 270 260 235 251 240280 271 consistencyNote 1)Unit of dynamic viscosity of base oil: mm²/s (40° C.)(1) White Flaking Life Test

Each of the above test greases was charged in an amount of 2.3 g into asingle row deep groove ball bearing fitted with a contact rubber sealhaving an inner diameter of 17 mm, an outer diameter of 47 mm, and awidth of 14 mm to prepare a test bearing. Then, the test bearing wascontinuously rotated under conditions of an inner ring rotation speed of10500 min⁻¹, room temperature atmosphere, and a radial load of 1320 N.When flaking was induced on the running surface of the outer ring togenerate vibration, the rotation was stopped and time passed until thattime was measured. The test was repeated five times per each of the testgreases and an average value was regarded as white flaking life. Theresults are shown in FIG. 2, the results being shown as relative valuesto the white flaking life of the test grease of Comparative Example 2.

From FIG. 2, among the test greases wherein a diurea compound is used asa thickening agent and zinc naphthenate and a succinate ester wereblended, there is a tendency that white flaking life increases as thedynamic viscosity of the base oil increases and it is found that thelife is almost saturated in the range exceeding 50 mm²/s (40° C.).Moreover, when the dynamic viscosity of the base oil is less than 20mm²/s (40° C.), an improving effect on the white flaking life is hardlyobserved.

(2) Dynamic Torque Test

Each of the above test greases was charged in an amount of 3.4 g into asingle row deep groove ball bearing fitted with a non-contact rubberseal having an inner diameter of 25 mm, an outer diameter of 62 mm, anda width of 17 mm (see FIG. 1) to prepare a test bearing. Then, dynamictorque was measured when the test bearing was continuously rotated for30 minutes under conditions of an inner ring rotation speed of 3600min⁻¹, a bearing temperature of 30° C., a radial load of 30 N, and anaxial load of 60 N.

The results were shown in FIG. 3 and it is found that an acceptancecriterion of 0.1 N·m or less is satisfied when the dynamic viscosity ofthe base oil falls within the range of 50 mm²/s (40° C.) or less

(3) Seizure Life Test

Each of the above test greases was charged in an amount of 2.0 g into asingle row deep groove ball bearing fitted with a non-contact rubberseal having an inner diameter of 25 mm, an outer diameter of 62 mm, anda width of 17 mm (see FIG. 1) to prepare a test bearing. Then, the testbearing was continuously rotated under conditions of an inner ringrotation speed of 10000 min⁻¹, a bearing temperature of 120° C., and aradial load of 98 N. When the outer ring temperature reached 150° C. dueto seizure, the rotation was stopped and time passed until that time wasmeasured. The test was repeated four times per each of the test greasesand an average value was regarded as seizure life. The results are shownin FIG. 4, the results being shown as relative values to the seizurelife of the test grease of Comparative Example 1.

From FIG. 4, it is found that seizure life increases as the dynamicviscosity of the base oil increases and the life is almost saturated inthe range exceeding 50 mm²/s (40° C.). Moreover, when the dynamicviscosity of the base oil is less than 20 mm²/s (40° C.), an improvingeffect on the seizure life is hardly observed.

From the above test results, it is found that the grease compositionsobtained by adding an urea compound as a thickening agent and zincnaphthenate or a succinate ester as an antirust additive to a base oilhaving a dynamic viscosity of 20 to 50 mm²/s (40° C.) according to theinvention hardly induces white texture flaking and seizure at a hightemperature and exhibits a low torque.

(4) Verification of Blending Amount of Antirust Additive

A base grease containing 13% by weight of a diurea compound(R₁/(R₁+R₃)=0.3) and 1% by weight of ZnDTC in a poly-α-olefin having adynamic viscosity of 40 mm²/s (40° C.) was prepared and zinc naphthenatewas added to the base grease with changing the amount of the salt to beadded, whereby test greases were obtained. Then, each of the testgreases was charged in an amount of 2.3 g into a single row deep grooveball bearing with a contact rubber seal having an inner diameter of 17mm, an outer diameter of 47 mm, and a width of 14 mm to prepare a testbearing and flaking generation probability was determined using a testapparatus shown in FIG. 5. In this connection, the test apparatus shownin the figure has a constitution that the inner ring of a test bearing75 was fitted to an edge of a shaft 70 supported by one pair ofsupporting bearings 71, 71, furthermore its outer ring is fixed to aholder 72, and rotation from a motor (not shown in the figure) istransmitted to the test bearing 75. The test was conducted under thesame conditions as in the above (1) white flaking life test.

Moreover, the same test grease was charged in an amount of 2.3 g into asingle row deep groove ball bearing with a contact rubber seal having aninner diameter of 17 mm, an outer diameter of 47 mm, and a width of 14mm to prepare a test bearing. After it was rotated at 1800 min⁻¹ for 1minute, 0.5 mL of 0.5% by weight of saline was introduced into thebearing and then the bearing was rotated at 1800 min⁻¹ for another 1minute. After the test bearing was allowed to stand under conditions of52° C. and 100% RH for 48 hours, the test bearing was dismantled and arusting state of each track surface of the inner and outer rings wasobserved. Evaluation was conducted according to the following standardsand a score of 2 or less was regarded to be acceptable.

<Rust Evaluation Score>

1: no rust

2: three small rust points or less

3: four small rust points or more

Results of the flaking generation probability and rust evaluation scoreare shown in FIG. 6. It is found that the generation of the whitetexture flaking and the rust generation are both suppressed when theamount of zinc naphthenate added falls within the range of 0.1 to 10% byweight.

(5) Verification of Blending Amount of Organometallic Salt

A base grease containing 13% by weight of a diurea compound(R₁/(R₁+R₃)=0.3), 1% by weight of zinc naphthenate, and 1% by weight ofa succinate ester in a poly-α-olefin having a dynamic viscosity of 40mm²/s (40° C.) was prepared and ZnDTC was added to the base grease withchanging the amount thereof to be added, whereby test greases wereobtained. Then, the flaking generation probability was determined in thesame manner as above. Furthermore, the above (3) seizure life test wasconducted.

Results of the flaking generation probability and rust evaluation scoreare shown in FIG. 7 and it is found that the white texture flakinggeneration is suppressed when the amount of zinc naphthenate added fallswithin the range of 0.1 to 10% by weight and the seizure life is alsoimproved at the same time.

INDUSTRIAL APPLICABILITY

As described in the above, according to the invention, there is obtaineda grease composition and rolling device having a good antirust property,suppressing generation of white texture flaking, and further havingexcellent anti-seizure performance.

1. A grease composition comprising: a base oil having a dynamicviscosity at 40° C. of 20 to 50 mm²/S; a urea compound as a thickeningagent in an amount of 8 to 30% by weight with respect to total weight ofthe grease; at least one antirust additive selected from carboxylicacids, carboxylate salts, and ester-based antirust additives in anamount of 0.1 to 10% by weight per single additive with respect to totalweight of the grease and in an amount of 0.1 to 15% by weight in totalof the additive with respect to total weight of the grease.
 2. Thegrease composition according to claim 1, wherein the antirust additiveis at least one selected from naphthenate salts and succinic acidderivatives.
 3. The grease composition according to claim 1, wherein thethickening agent is a mixture of diurea compounds represented by thefollowing general formulae (1) to (3):

wherein R₁ is a cyclohexyl group or an alkylcyclohexyl group having 7 to12 carbon atoms, R₂ is a divalent aromatic ring-containing hydrocarbongroup having 6 to 15 carbon atoms, and R₃ is an alkyl group having 8 to20 carbon atoms, wherein a ratio of [number of moles of R₁/(number ofmoles of R₁+number of moles of R₃)] is from 0.1 to 1.0.
 4. The greasecomposition according to claim 1, wherein the grease composition furthercomprising at least one selected from DTC or DTP-base additives andashless dithiocarbamates in amount of 0.1 to 10 by weight with respectto total weight of the grease.
 5. A rolling device into which the greasecomposition according to claim 1 is charged.